Control of corrosion in reaction vessels



Feb. 26, 1957 B. c. RAYNES ET AL v CONTROL OF CORROSION IN REACTION VESSELS Filed April" 29, 1955 H m .I

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ZZ HEATER INVENTORS BERTRAM QRAYNES :IYDWARD LTHELLMANN FIGZ,

United States Patent CONTROL OF CORROSION 1N REACTION VESSELS Bertram C. Haynes, Euclid, and Edward L. Thellmann,

Parma, Ohio, assignors, by mesne assignments, to Horizons Titanium Corporation, Princeton, N. 3., a corporation of New Jersey Application April 29, 1955, Serial No. 504,952

5 Claims. (Cl. 204-64) This invention relates to an improved design for vessels in which highly corrosive materials are to be processed or reacted at elevated temperatures. The principles herein disclosed may be readily adapted to the design of the container for highly heated corrosive liquids generally. More particularly, this invention may be utilized to advantage in the construction of cells intended to be employed in the electrolytic recovery of metals by a fused salt electrolysis.

For the purpose of illustration, this invention will be described as applied to the recovery of transition metals, but it will be appreciated that this is not to be taken as a limitation thereon since similar considerations will apply to other specific processes for the recovery of other products by fused salt electrolysis or by fused salt reactions.

In the recovery of transition metals, one of the more promising procedures which has been intensively investigated is electrolysis of molten salts. It has been found that most of the ordinary materials of construction are rapidly attacked by the corrosive molten salts constituting the fused bath, or by such other products of the electrolysis' as elemental or combined fluorine or chlorine. The attack is particularly severe since the operating temperatures are often as high as 900 C. The choice of a material of construction has often been found therefore to be limited to commercial graphite or carbon. Because of the highly corrosive nature of the fused salt and because of the porosity of commercial carbon crucibles, particularly in large sizes, a definite limit is thus placed upon cell size, and therefore cell capacity.

The prior art has long been aware of the possibility of increasing the life of furnace refractories-by permitting some of the contents of the furnace to freeze and to form a protective frozen shell which thereafter serves as a container for the corrosive molten contents of the furnace. In applying such a teaching to an electrolytic cell, it is necessary to sacrifice the advantages attendant on the use of the container or crucible as one of the electrodes, since the frozen salt shell generally does not conduct electricity. Thus, in prior art electrolytic cells, additional electrodes of relatively limited area must be provided to replace the container as an electrode.

By the practice of this invention, it now becomes pos sible to conduct the electrolysis in a frozen salt shell, while at the same time utilizing the container or crucible as one electrode. As a result, it becomes possible to conduct the desired electrolysis of a fused bath in a container which is nowise limited as. to size or'capacity, except for the strength of the materials employed in its construction, while at the same time preserving the purity of the electrolytic product obtained. Furthermore, because of the virtually limitless size of the electrolytic cell, much higher electrical charges may be used in the electrolysis.

These and other advantages will become apparent from the following specifications and claims in which? Figure 1 is a schematic representation of an electrolytic Patented Feb. 26, 1957 cell constructed in accordance with this invention seen in section with the cover removed and before the charge is present.

Figure 2 is a schematic representation of a similar section of the vessel containing a molten charge.

As shown in Figure 1, an electrolytic cell constructed in accordance with this invention includes an outer shell 10 of any suitable structural material, the principal function of which is to support and contain the materials within its confines. Generally, shell 10 is of metal such as steel. Surrounding shell 10 is a heat exchange means 12 for controlling the temperature of shell 10 within any desired range. The heat exchange means may comprise a plurality of perforated pipes by means of which heated or cooled fluid is continuously sprayed on shell 10, or it may comprise a coil disposed in intimate contact with shell 10, through which a heated or cooled fluid is caused to circulate. Any suitable means may be provided as the heat exchanger.

Within shell 10 there is disposed a container 14 constructed of carbon or graphite or other suitably inert, electrically conductive material. The container 14 serves as one of the electrodes in the preferred use of the cell shown. Where the vessel constructed in accordance with the invention herein disclosed is to be employed as a reaction vessel, not involving any electrolysis, the inner container 14 may be constructed of an electrically nonconductive material such as a ceramic, without sacrificing certain other benefits obtained in the practice of our invention.

Because, in accordance with this invention, a certain amount of leakage may be tolerated without a failure of the cell, the inner crucible 14 may be constructed of pieces of graphite or carbon or other material. These pieces need only fit reasonably closely together as they are not required to form a leakproof impervious vessel. In this way vessels of any size may be fabricated from relatively small, standard shapes much in the same man ner that carbon shells have been installed in the lower portions of blast furnaces. Common sizes for present day carbon blocks are pieces slightly larger than 1 foot by 2 feet in section by three to five feet in length.

Between the inner crucible 14 and the outer metallic shell 10, there is disposed a thermally insulating material 16 such as carbon black, lamp black, or powdered carbon or graphite, or even powdered metal, or other material having the required inertness toward the contents of crucible 14 and the desired relatively poor thermal conductivity.

Crucible 14 is adapted to contain a melt 13 composed essentially of one or more molten salts. In the electrolytic production of a transition metal one such bath migh contain one or more alkali metal halides or alkaline earth metal halides in combination with various transition metal compounds as described in U. S. Patents 1,815,654; 1,835,025 and 1,861,625 among others. Suspended in the bath are one or more cathodes 20 and one or more immersion type heaters 22 for maintaining the bath molten.

The above cell is constructed in the following manner:

After the volume required of vessel 14 and the temperatures at which bath 18 and outer shell 10 are to operate have been ascertained, the thickness of thermal insulation is computed to provide a zone intermediate of shell 10 and crucible 14 in which a temperature will be obtained corresponding to the temperature at which the molten material in vessel 14 becomes a solid.

To assemble the apparatus, shell 19 is positioned on a suitable support and a layer of thermal insulation of the desired thickness is provided in the bottom of shell 10. The thermal insulation may be disposed in the form of a loose powdery or granular material, or it may be tamped in place, or formed into a plastic mixture with a temporary plasticizer such as water or methylcellulose and poured in place and baked. Next a bottom is constructed for vessel 14 from standard flat shapes of carbon, graphite or other suitably inert materials which may be interlocked if desired. Gradually, the sides of vessel 14 are built up and the space between vessel 14 and shell is gradually filled with insulation. Heat exchanger 12 is then positioned adjacent shell 10 and, as shown in Figure 2, a heating means 22 and one or more electrodes are positioned within the vessel 14. Means are provided to connect electrodes 20 and vessel 14 to a source of potential, in the event that it is desired to conduct an electrolysis in the vessel.

In operation, once the components have been assembled, the bath or reaction mixture is melted by means of heaters 22 positioned within the vessel. When molten as shown in Figure 2, the contents of vessel 14 tend to leak either through any pores or discontinuities in the walls of the vessel into the thermal insulation 16. Heat exchanger means 12 is operated so as to restrict the leakage to a region corresponding to a zone 15 therein by adjusting the heat transfer so that a portion of the liquid becomes solid in a zone 17 after leaking through crucible 14. in this manner, the entire contents of vessel 14 remains molten throughout the vessel and the vessel may properly function as one electrode in any electrolysis conducted therein.

The above described cell is in marked contrast with prior art cells operated heretofore with a lining formed of frozen salts. Since thesolidified salt is non-conductive, it is necessary to insert an anode and a cathode in the bath, in order to conduct an electrolysis. As shown in the figure, the frozen lining and the anode and cathode occupy a considerable traction of the volume of crucible and thus diminishes the eificiency of the operation. Furthermore, the area of both the anode and cathode is but a small fraction of the area of crucible 14 of Figure 1 and hence, the electrical capacity of such cells is substantially less than that of the cell of Figure l, of equal volume.

A further precaution concerning the construction and operation of vessels as described above should be mentioned. When operating at temperatures at which the carbonaceous materials employed tend to oxidize, or in instances in which the contents of the vessel tend to react detrimentally with the normal atmosphere, it will be appreciated by those skilled in the art that precautions must be taken to prevent such undesirable developments. Usually it sufiices to provide a cover 24 extending over the vessel, whereby a controlled atmosphere of any desired composition may be maintained above the contents of the vessel and in contact with the exposed portions of the inner vessel 14. Cover 24 may be provided with openings through which heating means 22, electrodes 20, connections 26 to a source of a suitable atmosphere and the i like, may be positioned.

We claim:

1. An electrolytic cell comprising: an outer supporting shell, an inner electrically conductive slightly porous shell spaced at least a predetermined minimum distance from the outer shell and adapted to contain a body of electrolyte, heating means positioned in the electrolyte contained in the inner shell, thermal insulation filling the volume between the outer shell and the inner shell, at least one electrode positioned within the inner shell in contact with the electrolyte, a source of potential, leads connecting the source of potential with the inner shell and with the said electrode whereby an electrolysis may'be conducted in the inner shell and means to maintain the outer supporting shell at a temperature sutficiently below the melting temperature of the electrolyte thatthere are provided three zones in the thermal insulation between the inner shell and the outer shell as follows: a first zone adjacent the inner shell and containing some electrolyte in liquid form, a second zone adjacent the outer shell conell) taining only thermal insulation and a third zone between the two in which any electrolyte present is in solid form.

2. An electrolytic cell comprising: an outer supporting shell, an inner electrically conductive slightly porous shel spaced at least a predetermined minimum distance from the outer shell and adapted to, contain a body of molten salt, heating means positioned in the molten salt con tained in the inner shell for maintaining the salt in molten condition, thermal insulation filling the volume between the outer shell and the inner shell, at least one electrode positioned within the inner shell in contact with the molten electrolyte, a source of potential, leads connecting the source of potential with the inner shell and with the said electrode whereby an electrolysis may be conducted in the inner shell, and means to maintain the outer supporting shell at a temperature sufilciently below the melting temperature of the electrolyte that there are provided three zones in the thermal insulation between the inner shell and the outer shell as follows: a first zone adjacent the inner shell and containing some electrolyte in liquid form, a second zone adjacent the outer shell containing only thermal insulation and a third zone between the two in which any electrolyte present is in solid form.

3. An electrolytic cell comprising: an outer supporting shell, an inner electrically conductive slightly porous shell of carbonaceous material spaced at least a predetermined minimum from the outer shell and adapted to contain a fused salt electrolyte, heating means positioned within the fused salt for maintaining said salt in molten condition, thermal insulation filling the volume between the outer shell and the inner shell, at least one electrode positioned within the inner shell in contact with the electrolyte, a source-of potential, leads connecting the source of potential with the inner shell and with the said electrode whereby an electrolysis may be conducted in the inner shell and means to maintain the outer supporting shell at a temperature sufiiciently below the melting temperature of the electrolyte that there are provided three zones in the thermal insulation between the inner shell and the outer shell as follows: a first zone adjacent the inner shell and containing some electrolyte in liquid form, a second zone adjacent the outer shell containing only thermal insulation and a third zone between the two in which any electrolyte present is in solid form.

4. A method of containing corrosive liquid materials which comprises: providing a porous inert container for corrosive liquid, establishing therein a body of liquid corrosive material, maintaining the body of liquid at an elevated temperature by a heating means positioned within the body of liquid, disposing an outer support shell spaced from the inner container, introducing a comminuted thermally insulating material into the space between the outer shell and the container and maintaining the temperature of the outer shell so as to establish three zones in the thermal insulation as follows: a first zone adjacent the inner container in which the corrosive material is present in liquid form in the thermal insulation, a second zone adjacent the outer shell in which the thermal insulation alone is present, and an intermediate zone between the first Zone and the second Zone in which the corrosive material is present in the form of a solid.

5. A method of electrolyzing a corrosive fused electrolytic salt bath which comprises: providing a porous inert electrically conductive container for the salt bath, es-

tablishing therein a fused salt bath of the corrosive electrolyte, maintaining the fused salt bath in fused condition by heating means positioned within the bath. disposing an outer support shell spaced from the inner container, introducing a comminuted thermally insulated material into the space between the outer shell and the container, immersing at least one electrode in the fused salt bath, impressing a potential through the salt bath by suitably connecting the immersed electrode and the container to a source of potential whereby the fused electrolyte is electrolyzed and maintaining the temperature of the outer References Cited in the file of this patent shell during the electrolysis of the fused salt electrolyte so as to establish three zones in the thermal insulation UNITED STATES PATENTS as follows: a first zone adjacent the inner conductive con- 93 5,796 V Kugelgen at 1 O t, 5, 1909 tainer in which the electrolyte is present in liquid form, 5 2,311,257 Sawyer t at F b, 16, 1943 a second zone adjacent the outer shell in which thermal insulation alone is present and an intermediate zone be- FOREIGN PATENTS tween the first zone and the second zone in which the 366,523 France July 30, 1906 electrolyte salts are present in solid form. 

5. A METHOD OF ELECTROLYZING A CORROSIVE FUSED ELECTROLYTIC SALT BATH WHICH COMPRISES: PROVIDING A POROUS INERT ELECTRICALLY CONDUCTIVE CONTAINER FOR THE SALT BATH, ESTABLISHING THEREIN A FUSED SALT BATH OF THE CORROSIVE ELECTROLYTE, MAINTAINING THE FUSED SALT BATH IN FUSED CONDITION BY HEATING MEANS POSITIONED WITHIN THE BATH, DISPOSING AN OUTER SUPPORT SHELL SPACED FROM THE INNER CONTAINER, INTRODUCING A COMMINUTED THERMALLY INSUTLATED MATERIAL INTO THE SPACE BETWEEN THE OUTER SHELL AND THE CONTAINER IMMERSING AT LEAST ONE ELECTRODE IN THE FUSED SALT BATH IMPRESSING A POTENTIAL THROUGH THE SALT BATH BY SUITABLY CONNECTING THE IMMERSED ELECTRODE AND THE CONTAINER TO A SOURCE OF POTENTIAL WHEREBY THE FUSED ELECTROLYTE IS ELECTROLYZED AND MAINTAINING THE TEMPERATURE OF THE OUTER SHELL DURING THE ELECTROLYSIS OF THE FUSE SALT ELECTROLYTE SO AS TO ESTABLISH THERE ZONE IN THE THERMAL INSULATION AS FOLLOWS: A FIRST ZONE ADJACENT THE INNER CONDUCTIVE CONTAINER IN WHICH THE ELECTROLYTE IS PRESENT IN LIQUID FORM, A SECOND ZONE ADJACENT THE OUTER SHELL IN WHICH THERMAL INSULATION ALONE IS PRESENT AND AN INTERMEDIATE ZONE BETWEEN THE FIRST ZONE AND THE SECOND ZONE IN WHICH THE ELECTROLYTE SALTS ARE PRESENT IN SOLID FORM. 